In past years many operations of agricultural vehicles have been automated, which previously were controlled manually by an operator. In order to reduce the work load on the operator, for example, various steering systems have been proposed that automatically steer the vehicle across a field. They can reduce the work load on the operator and make it possible for the operator to concentrate on other important aspects of the work, such as a control of the load on the operating arrangements of the vehicle. In the future it may also be conceivable that such vehicles could be operated across a field without an operator.
Automatic steering systems have been described which rely on various principles. Systems operating without contact can be divided into systems based an a measurement of distance that attempt to detect the difference in height between the harvested crop that has been cut and the crop that has not been cut and systems based on a picture image that attempt to detect the difference in appearance between the harvested crop that has been cut and the crop that has not been cut at the side of the edge of the harvested crop or the difference between the plants and the ground.
For example, systems based on a measurement of distance can be found in EP 0 887 660 A and EP 1 271 139 A.
EP 0 887 660 A describes a measuring arrangement in which a laser beam directed downward towards the ground is pivoted across a range of angles. On the basis of its direction and position, the location of the point is determined at which the laser beam is being reflected. In this way a profile of the height of the ground is determined that makes it possible to recognize a swath or an edge of the harvested crop at the border between the part of the field of a cereal crop that has been harvested and the part remaining standing and thereby to generate a measurement signal.
In a laser distance measuring arrangement of this type EP 1 271 139 A proposes in addition that the amount of the crop to be taken up can be determined by the intensity of the reflected laser radiation.
A disadvantage of the systems based on the measurement of distance lies in the fact that a relatively costly distance measuring arrangement is required with a laser and an electronic arrangement for the determination of the elapsed running time of the laser beam.
Examples of systems based on the use of a picture image that employ a camera directed at the field and an electronic picture processing arrangement can be found in DE 35 07 570 A, EP 0 801 885 A, WO 98/46065 A, WO 96/17279 A, JP 01 319 878 A, JP 03 012 713 A, U.S. Pat. No. 6,278,918 B, U.S. Pat. No. 6,285,930 B, U.S. Pat. No. 6,385,515 B, and U.S. Pat. No. 6,490,539 B.
DE 35 07 570 discloses an automatic steering system for an agricultural tractor that is intended to operate along or across a field with plants arranged in rows. The system derives the orientation of the rows from a picture image provided initially. Then the picture is processed along lines that extend parallel to an established orientation and the average grey scale values for each row are stored in memory. During the operation across the field, the field is processed along the same lines. The result of the processing of each line are compared with the stored values and the reference values are maintained by appropriate steering signals.
EP 0 801 885 A describes an agricultural machine with a camera whose picture image is digitized and subjected to a picture imaging process. Here each of the rows is examined for its position in a step function in which the grey scale value changes step-by-step. In the individual rows the pixel positions are determined for the greatest likelihood of the step function. These positions are utilized to generate a steering signal.
Furthermore WO 98/46065 A recognizes on the basis of the information contained in the various rows whether they contain a border of the harvested crop. On the basis of the information of all rows, the row at the end of the field to be harvested is established.
WO 96/17279 A proposes that a border be superimposed upon a picture taken by a camera (such as a parallelogram) and to identify objects within that shape by a brightness that lies above a threshold value. The objects are subjected to a regression analysis in order to determine their offset to a previously established row, which in turn is again used for the generation of a steering signal.
JP 01 319 878 A concerns a steering system in which areas of specific colors are connected to each other in a picture taken by a camera, in order to define a straight line. This line is regarded as a plant row and is used to generate a steering signal.
JP 03 012 713 A proposes an automatic steering system for a tractor in which a window of a picture that was taken by a camera is defined. In this window a virtual line is defined along which the plants are arranged. Here the picture is compared with a virtual plant row, in order to generate a steering signal.
U.S. Pat. No. 6,278,918 B proposes that the crop to be taken up be divided into two or four regions. In these regions the location and the direction of the plant row is detected. The plant row that can be recognized better is used to generate the steering signal. The decision whether or not a pixel represents a plant is performed by K-means clustering in which a histogram of the pixel values is generated, in which the x-axis represents the levels of the grey scale values and the y-axis represents the number of pixels with the particular grey scale values. The pixels that more likely represent the plants therefore appear at the right side of the histogram. The algorithm divides the histogram generated by the picture from the camera into a predetermined number of classes K, whose intervals are spaced equally. The data are processed recursively in order to find the average grey scale value of the class. Based on a predetermined matrix or so as to minimize a cost function, the borders between classes are shifted recursively until the centers of the classes are shifted by less than a given tolerance. Following that a threshold is placed to differentiate between the plants and the ground at the beginning of the class K or another class. The classification can also be performed by other clustering algorithms such as dividing processes, self-organizing maps and neuronal logic or fuzzy logic. U.S. Pat. No. 6,285,930 B, U.S. Pat. No. 6,385,515 B and U.S. Pat. No. 6,490,539 B also refer to this system.
In the systems described that are based on a picture image, the distinction is made on the basis of the brightness and/or the color of the individual pixels whether or not they represent a plant. In some of these systems the plants that have been identified are connected by a virtual line that is used to generate a steering signal. However in some cases in which automatic steering of an agricultural machine is desirable, it is not easy or almost impossible to recognize on the basis of the color or the brightness whether or not a pixel belongs to a line that should be followed. A typical example is a swath of grass that has been mowed or straw lying on a field of a cereal crop that has been harvested. The swath consists of the same material as the stubble remaining standing on the field and therefore in many cases has more or less the same brightness and color as the swath. The systems described that are based on a picture image can be applied here only with difficulty. Other systems described recognize plants on a field and orient themselves by the rows described, in order, for example, to apply chemicals. In case that no plants are present these systems cannot be applied.
In the description of EP 0 801 885 A (page 3, first paragraph) a possible process is mentioned which uses a local, two-dimensional Fourier transformation operator as basis for a texture-based segmentation in order to establish a special frequency band with a significant difference between plants that have been cut and those remaining standing. The fact is cited that this process did not show any clear indication of a significant difference in its initial tests. For that reason it was abandoned.
The problem underlying the invention is seen in the need to make available a process for automatic steering of an agricultural machine and a corresponding steering system, that operates reliably and can be attained at reasonable cost.